1. Field of the Invention
The present invention broadly relates to telecommunications. More particularly, the present invention relates to the conducting of diagnostic tests utilizing an OAM loopback cell in an ATM network.
2. State of the Art
Perhaps the most awaited, and now fastest growing technology in the telecommunication field in the 1990's is known as Asynchronous Transfer Mode (ATM) technology. ATM is providing a mechanism for removing performance limitations of local area networks (LANs) and wide area networks (WANs) and providing data transfers with at a speed of on the order of terabits/second. The variable length packets of LAN and WAN data are being replaced with ATM cells which are relatively short, fixed length packets. Because ATM cells can carry voice, video and data across a single backbone network, the ATM technology provides a unitary mechanism for high speed end-to-end telecommunications traffic.
In order for ATM technology to develop, it must be functional in its own right as well as compatible with existing technology. To be compatible, on the one hand, the ATM cells must be capable of receiving and accommodating voice, video, and LAN and WAN type data; while on the other hand, ATM cells must be capable of adapting to high speed technology such as the synchronous optical network (SONET). In order to meet these and other requirements, a technical group called the ATM Forum which is comprised of numerous corporate representatives has been proposing ATM "standards" which are being provided to ANSI and the ITU-T for their consideration and adoption. Details of those standards may be found in proceedings of the ATM Forum. Of particular interest among the ATM Forum, ANSI and ITU-T documents is a document which is incorporated by reference in its entirety herein and which is dated November 1995 and entitled "B-ISDN Operation and Maintenance Principles and Functions, ITU-T Recommendation I.610 .
As is set forth in Section 2 of Draft Recommendation I.610, Operation, Administration, and Management (OAM) cells provide performance monitoring, defect and failure detection, system protection, defect information, and fault localization functions. Performance monitoring is a function which processes user information to produce maintenance information specific to the user information. The maintenance information is added to the user information at the source of a connection/link and extracted at the sink of a connection/link. Analysis of the maintenance event information at the connection sink allows analysis of the transport integrity. Defect and failure detection is accomplished by continuous or periodic checking and results in the production of various alarms. In the system protection function, the effect of a defect on the transport of user information is minimized by blocking or changeover to other entities, and the failed entity is excluded from operation in order to protect the system. Response to status report requests will also be Given. Fault localization involves a determination by internal or external test systems of a failed entity if defect information is insufficient.
A desirable mechanism for fault localization, pre-service connectivity verification, and on-demand connectivity monitoring is the use of an OAM loopback cell. The ATM layer loopback capability allows for OAM loopback cells to be injected at one location, along a VPC or VCC, and returned at a different location, without having to take the connection out of service. A loopback cell can be injected at any accessible point along the VPC or VCC (i.e., at either end-point or at any connecting point). This cell is looped back at a downstream point specified by the information contained in the OAM cell. The downstream point is either a segment end-point or a connection end-point.
The source point (injection point) sends forward loopback cells. As is discussed in more detail below, the loopback point changes the cell into a backward cell and sends it back upstream. There is a correlation tag (random thirty-two bit field) used to verify the transmitted forward cell against the received backward loopback cell. According to the ITU-T I.610 standard, the default waiting time between the transmission of successive loopback cells on a VPC or VCC is five seconds. The loopback will be considered unsuccessful if the loopback cell is not returned to the originating point within the standard default waiting time of five seconds.
The ATM loopback capabilities are defined in Sections 6.2.1.1.3 (VP Loopback Capability), 6.2.2.1.3 (VC Loopback Capability), and Annex C (Procedures to be performed when receiving Loopback OAM cells), while the format of the loopback OAM cell itself is defined in Section 7.2.4 (Loopback Cell). As set forth in Section 7.1 of the ITU-T Recommendation, the ATM layer OAM cell includes five bytes of header followed by a forty-eight byte OAM Cell Information Field. Details of the five bytes of header include are set forth in Recommendation I.361 which is hereby incorporated by reference herein in its entirety. Recommendation I.361 states that for F4 flow identification, two pre-assigned VCIs are used to distinguish OAM cells meant for VPC end-to-end or VPC segments. These two values are defined in Recommendation I.361. For F5 flow identification, two PTI values are used to distinguish OAM cells for VCC end-to-end or VCC segments. These two values are defined in Recommendation I.361. Turning to the forty-eight bytes OAM Cell Information Field, the first four bits define the OAM cell type; e.g., fault management, performance, management, and activation/deactivation. The next four bits identify the OAM function type; i.e., the type of function performed by the cell within the management type. Following the four bit OAM function type field, forty-five bytes of information pertinent to the function type are sent, followed by a six bit reserved field for future use, and a ten bit error detection code field.
As set forth in Section 7.2.4, the forty-five byte OAM Cell Information field format for a VP/VC end-to-end and segment level loopback is provided (as seen in prior art FIG. 1) to include a one byte loopback indication field, a four byte correlation tag field, a sixteen byte loopback location ID field, an optional sixteen byte source ID, and eight bytes of unused data. According to the standard, the loopback identification field uses the least significant bit as a Boolean indication as to whether or not the cell has already been looped back. Thus, the source point encodes this field as 00000001, while the loop back point changes the encoding to 00000000, thereby avoiding the problem of an infinite loopback. The four-byte correlation tag field is used upon receiving a OAM cell having a 00000000 value in the loopback identification field to correlate the transmitted cell with the received OAM cell; i.e., only upon receiving an OAM cell with a 00000000 value in the loopback identification field will a determination be made as to whether the identification field corresponds to the value placed by the transmitting device into the correlation tag field. The sixteen byte loopback location ID field identifies as an option the connecting point along the virtual connection or connection segment where the loopback is to occur. The value of this field is not subject to standardization and encoding of non-default values (i.e., values other than all ones) in this field is optional. Finally, the sixteen byte optional source ID field can be used to identify the source originating the loopback cell. Again, the default value is all ones, and the value of the field is not subject to standardization.
While the loopback mechanism for OAM cells as defined in Recommendation I.610 works properly in many situations, it has been recognized by the inventors that there exists some situations where a looped back OAM cell will not be identified by the source originating the loopback cell as its own originated cell. In particular, where a physical loopback exists in the network, the OAM loopback test on that connection will fail due to the fact that, although an OAM loopback cell is returned, it is not marked as a "backward" cell; i.e., the value in the loopback identification field will be 00000001 instead of 000000000.